Calcium carbide residue: Alkaline activator for clay–fly ash geopolymer

• Industrial by-product: Calcium Carbide Residue (CCR).
• A green geopolymer subgrade using CCR as alkaline activator.
• Strength and microstructural analysis of CCR–FA based geopolymer.
• Role of heat temperature and curing on strength development.

Calcium Carbide Residue (CCR) and Fly Ash (FA) are waste by-products from acetylene gas and power plant production, respectively. The liquid alkaline activator studied in this research is a mixture of sodium silicate solution (Na2SiO3), water and CCR. The primary aim of this research is to investigate the viability of using CCR, a cementitious waste material, as an alkaline activator and FA as a precursor to improve the engineering properties of a problematic silty clay to facilitate its usage as stabilized subgrade material. The influential factors studied are Na2SiO3/water ratio, FA replacement ratio, curing time, curing temperature and soaking condition for a fixed CCR content of 7%. Strength development is investigated via the unconfined compression test. Scanning Electron Microscopy (SEM) observation is used to explain the role and contribution of influential factors on strength development. CCR dissolves the silicon and aluminum in amorphous phase of FA and the Na2SiO3 acts as a binder. The maximum soaked strength of the clay-FA geopolymer is found at Na2SiO3/water ratio of 0.6 and FA replacement ratio of 15%. The optimal Na2SiO3/water ratio is approximated from index test, which is a very practical approach. The clay-FA geopolymers with 40 °C curing exhibit higher strength than those with room temperature curing, indicating the possibility of using clay-FA geopolymer for pavement subgrade applications. The 7-day soaked strength at the optimal ingredient meets the strength requirement for subgrade materials specified by the local national road authority. CCR is found to be a sustainable alkaline activator for geopolymer stabilized subgrade materials, which will result in the diversion of significant quantities of this by-product from landfills.